Levels 40–45: Networking

Level 40: IP Addressing

Command	What it does
`ip addr`	Show every IP address on every interface
`ip link show`	Show link-layer state (up/down, MAC)
`ip addr add 192.168.1.50/24 dev eth0`	Assign an address (temporary)
`ip link set eth0 up`	Bring an interface up
`hostname -I`	Quick own-IP check, script-friendly

Level 41: Routing & Gateways

Command	What it does
`ip route`	Show the routing table
`ip route add default via 192.168.1.1`	Set the default gateway
`ip route add 10.0.0.0/24 via 192.168.1.254`	Add a static route to a network
`traceroute google.com`	Show each hop to a destination
`mtr google.com`	Continuous traceroute + ping in one view

Level 42: DNS Tools

Command	What it does
`dig google.com`	Query a domain’s DNS record
`dig google.com MX`	Query a specific record type
`nslookup google.com`	Simpler, terser lookup
`cat /etc/resolv.conf`	Show configured DNS resolvers
`dig -x 8.8.8.8`	Reverse lookup, IP to hostname

Level 43: Firewalls

Command	What it does
`iptables -L -n`	List current rules, numeric output
`iptables -A INPUT -p tcp --dport 22 -j ACCEPT`	Allow SSH
`nft list ruleset`	List rules the modern way
`ufw allow 22`	Allow a port, the simple way
`ufw enable`	Turn the firewall on

Level 44: VLANs & Trunking

ip link add link eth0 name eth0.10 type vlan id 10   # VLAN subinterface for tag 10
ip link set eth0.10 up
ip addr add 10.10.10.5/24 dev eth0.10
ip -d link show eth0.10                               # confirm the VLAN tag
ip link delete eth0.10                                 # remove it

A trunk port carries multiple tagged VLANs over one physical link; an access port only ever carries one, untagged. The switch port facing a Linux VLAN subinterface needs to be a trunk carrying that tag.

Level 45: Bonding & Troubleshooting

Command	What it does
`tcpdump -i eth0`	Capture live traffic on an interface
`tcpdump -i eth0 port 443`	Filter the capture to one port
`cat /proc/net/bonding/bond0`	Check a bonded interface’s status
`ip link add bond0 type bond mode active-backup`	Create a bonded interface
`ip -s link show eth0`	Interface statistics: errors, drops